2 (g.d.b) Mid-Ocean Ridges

Mid-ocean ridges curve like the seam of a baseball along the sea floor. They extend into all of Earth’s oceans. Most of the mountains in the mid-ocean ridge system lie hidden under hundreds of meters of water.

What Is Sea-Floor Spreading? In sea-floor spreading, the sea floor spreads apart along both sides of a mid-ocean ridge as new crust is added. As a result, the ocean floors move like conveyor belts, carrying the continents along with them.Sea-floor spreading begins at a mid-ocean ridge, which forms along a crack in the oceanic crust. Along the ridge, molten material that forms several kilometers beneath the surface rises and erupts. At the same time, older rock moves outward on both sides of the ridge. As the molten material cools, it forms a strip of solid rock in the center of the ridge. When more molten material flows into the crack, it forms a new strip of rock.

Evidence for sea-floor spreading: Eruptions of molten material, magnetic stripes in the rock of the ocean floor, and the ages of the rocks themselves. Scientists found evidence that new material is indeed erupting along mid-ocean ridges and that molten material has erupted again and again along the mid-ocean ridge.Evidence From Magnetic Stripes: Earth behaves like a giant magnet, with a north pole and a south pole. Surprisingly, Earth’s magnetic poles have reversed themselves many times during Earth’s history. Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes.” These stripes hold a record of reversals in Earth’s magnetic field. The rock of the ocean floor contains iron. The rock began as molten material that cooled and hardened. As the rock cooled, the iron bits inside lined up in the direction of Earth’s magnetic poles. This locked the iron bits in place, giving the rocks a permanent “magnetic memory.” Scientists recorded the magnetic memory of rocks on both sides of a mid-ocean ridge. They found that stripes of rock that formed when Earth’s magnetic field pointed north alternate with stripes of rock that formed when the magnetic field pointed south. The pattern is the same on both sides of the ridge.Evidence From Drilling Samples: The final proof of sea-floor spreading came from rock samples obtained by drilling into the ocean floor. Then the scientists determined the age of the rocks in the samples. They found that the farther away from a ridge the samples were taken, the older the rocks were. The youngest rocks were always in the center of the ridges. This showed that sea-floor spreading really has taken place.